This invention relates to an electric power generation system of the kind employing both gas and steam turbines and their associated electric generators. It is known to supply the gas turbine with gas from a gasifier of the fluidized bed type in which low grade solid fuel is granulated, heated with refractory particles and `fluidized` by forcing air through the bed of material. The heating is controlled so that low grade fuel gas is produced which, after suitable cleaning and cooling, is supplied to the gas turbine where it is burnt with excess air and the combustion products used to power the turbine.
The gas turbine exhaust gases contain useful heat which can be extracted in a waste heat recovery boiler. Steam produced by this boiler is used to drive a steam turbine and its associated electric generator.
Further steam for the steam turbine can be produced by a circulating fluidized bed combustor which may be fed by char from the gasifier, supplementary coal, surplus gas produced by the gasifier, or any combination of these.
A problem arises in systems such as the above when sudden load variations occur on the electrical system fed by the turbogenerators. It is undesirable to run down the gas turbine in such circumstances since its efficiency suffers significantly with load variation. Load variations are therefore absorbed as far as possible by control of the steam turbine. The steam turbine can be run down rapidly but this results in surplus steam supply. A considerable portion of the available steam, say 30%, can, initially at least, be diverted to a steam condenser, bypassing the steam turbine in so doing. This is wasteful however, and further off-loading of the turbine cannot in any case be dealt with in this way. Reduction of steam generation can be achieved by diverting the gas turbine exhaust directly to atmosphere (via the `blast stack`) so bypassing the waste heat recovery boiler, or by reducing the gas supply to the circulating fluidized bed combustor. The former alternative is wasteful and the latter causes a gas surplus in the gasifier output.
It is difficult to turn the gasifier down to any significant extent without losing the ability to turn it up again rapidly. Turning down can be achieved by `slumping the spouting bed`, i.e. cutting off the air supply so that the bed is no longer fluidized. This effectively shuts down the gasifier with the result that resumption of electric power generation will take up to 6 hours. Such a situation is clearly to be avoided if at all possible.